

ELECTRICAL UNITS OF MEASUREMENT. 105 



think no more of the earth, so far as his measure- 

 ment of space is concerned. But how about .the 

 mean solar second, in terms of which he measures 

 his time ? What of it, if he has left the earth for 

 good ; or if, even without leaving the earth, he 

 carries on his scientific work on the earth through a 

 few million years, in the course of which the period 

 of the earth's rotation round its axis, and of its re- 

 volution round the sun, will both be very different 

 from what they are now ? If he takes a good 

 watch or chronometer with him, well rated before 

 he leaves the earth, it will serve his purpose as 

 long as it lasts. What it does is merely to count 

 the vibrations of a certain mass under the influence 

 of a certain spring (the balance-wheel under the 

 influence of the hair-spring). If, for any secular 

 experiment he has in hand, he wishes to keep up a 

 continuous reckoning of time, he must keep his 

 watch always going, and not a vibration will be 

 lost in the counting performed by the hands. But 

 if he merely wishes to keep his unit of time, and 

 to make quite sure that any number of million 

 years hence, this shall be within one-tenth per 



